1. Field of Invention
This invention relates to an automatic pole-zero adjustment circuit for an ionizing radiation spectroscopy system. Specifically, this invention relates to a circuit for an ionizing radiation spectroscopy system which automatically adjusts the pole-zero based upon peak shape.
2. Description of the Related Art
Radiation detection systems generally employ a radiation detector, such as a germanium or a scintillation detector, to detect radiation from a radiation source, such as alpha or gamma rays. The detection of such energy results in a charge pulse having an amplitude proportional to the energy of the incident radiation. The charge pulse is converted to a voltage pulse by a feedback capacitor incorporated in a preamplifier. A resistor is added in parallel with the capacitor in order to discharge the capacitor in a reasonable amount of time to allow for processing of successive pulses. The parallel combination of the capacitor and the resistor defines the time constant of the exponential decay of the trailing edge of the voltage pulse. The voltage pulse is submitted to a high pass filter to shorten the pulse and improve the signal-to-noise ratio. However, the high pass filter, when supplied with an exponentially decaying signal, produces a filtered signal with an undesirable undershoot, i.e., an excursion below baseline voltage. This is a serious problem because the radiation pulses arrive randomly in time and succeeding pulses can occur during the undershoot of a previous pulse. When this occurs, the measurement of the amplitude of the succeeding pulse is distorted. To compensate, a pole-zero cancellation circuit is used to eliminate the undershoot by diverting a portion of the voltage pulse at the high pass filter input around the filter and combining the diverted portion with the high pass filter output.
Various techniques have been used to implement the pole-zero cancellation method. This was originally done manually. (See Nowlin et al., "Elimination of Undesirable Undershoot in the Operation and Testing of Nuclear Pulse Amplifiers", Rev. Sci. Instr., vol.36, no. 2, December 1965, pp 830-839). However, untrained non-technical personnel, such as at medical clinics, encountered difficulty when compelled to adjust the shunting resistance or other components to null the undershoot and avoid overshoot. Accordingly, an automatic approach was proposed as in U.S. Pat. No. 4,866,400 (the '400 patent) to Britton, Jr. et al., issued on Sep. 12, 1989, entitled Automatic Pole-Zero Adjustment Circuit for an Ionizing Radiation Spectroscopy System, fully incorporated herein by reference. However, even using the automated approach, improvement of the adjustment accuracy is desirable by eliminating errors such as glitches, pedestals, offsets, and temperature drift in the analog automatic pole-zero (APZ) sampling circuit.
U.S. Pat. No. 5,872,363 issued to Bingham et al., discloses an automatic pole-zero adjustment circuit for an ionizing radiation spectroscopy system which directly measures the over/undershoot of the digital filtered signal. A correction signal is calculated based on the measured over/undershoot value and that signal is applied to a pole-zero adjustment network. Accordingly, Bingham et al., make automatic pole-zero adjustments based on intermediate outputs of the ionizing radiation spectroscopy system.
By making adjustments based upon the final output of the ionizing radiation spectroscopy system, a more accurate and stable solution for making automatic pole-zero adjustments is achieved over the prior art systems.
It is therefore an object of this invention to provide an improved automatic pole-zero adjustment circuit.
Yet another object of this invention is to provide such an improved automatic pole-zero adjustment circuit which is more accurate.
A still further object of this invention is to provide an improved automatic pole-zero adjustment circuit which is more flexible in the selection of sampling and correction circuits.